1. Field of the Invention
The present invention relates to a light-emitting apparatus having a semiconductor laser capable of switching the direction of the plane of polarization of output light between two orthogonal directions by controlling the injection current, and an optical communication system and a polarization modulation control method using this apparatus.
2. Related Background Art
Japanese Laid-Open Patent Application Nos. 62-42593 and 62-144426 have described conventional communication systems using a semiconductor laser (distributed feedback (DFB) semiconductor laser) which switches, by controlling the injection current, the direction of the plane of polarization of output light between so-called TE and TM modes which are perpendicular to each other. In these conventional systems, optical communications are performed by converting a change in the polarization plane into a change in the intensity by the use of a combination of a semiconductor laser which can switch the polarization mode of output light between TE and TM, and a polarization selecting means which transmits the output light of one of the two polarization planes.
In order to stably operate semiconductor lasers which are used in communications of this sort and by which the plane of polarization of output light is changed, APC (Automatic Power Control) that has been conventionally performed for semiconductor lasers is unsatisfactory when it is performed singly, so control for stabilizing the operating point of a semiconductor laser is required. One possible example of this control method is to separate a portion of the output light into TE polarized light and TM polarized light, convert these two light components into electrical signals, and control a semiconductor laser by using the two electrical signals.
Unfortunately, the above conventional method has the following drawbacks since the control operation is done by using a portion of the output light from a semiconductor laser which can change the plane of polarization of output light by controlling the injection current.
(1) Since a portion of the output light is used, an optical system for this purpose is necessary (the system is complicated).
(2) If this branching optical system is integrated with a semiconductor laser, an excess loss takes place (the loss increases).